Welding gun and methods conducted using the same

ABSTRACT

An intelligent welding gun is provided with a fixed side sensor in a fixed side portion. The mechanical impedance of the fixed side portion is kept small, which permits the mechanical impedance to be set in a range where the fixed side sensor can effectively detect at least one of a position of a fixed side welding tip and a pressing force imposed on the fixed side welding tip. The fixed side sensor and a moving side sensor constitute a redundant sensor measurement system. Various kinds of methods conducted using the above welding gun include a method of calibrating a sensor (including calibration of a reference point and a gain), a control method of suppressing a welding expulsion, a re-welding feedback control method, a control method of a welding strength, a control method of reducing a clearance between workpieces, a method of correcting a welding robot track, and a method of managing a positional accuracy change at a welding point.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an intelligent welding gunprovided with a sensor, and various kinds of methods conducted by usingthe welding gun including a calibration method of a sensor, a controlmethod of welding (which includes, for example, a control method of apressing force, a feedback control method of re-welding, a controlmethod of welding strength, a control method of suppressing thegeneration of a weld expulsion, and a control method of a track of awelding robot), and a managing method of a change in a positionalaccuracy of a welding point.

[0003] 2. Description of Related Art

[0004] Japanese Patent Publication No. 10-94882 discloses a method forcontrolling a pressing force of a welding gun. In the method, a smallquantity of an elastic displacement, generated in a fixed side electrodetip when a moving side electrode tip is driven to contact the fixed sideelectrode tip and further driven to press the fixed side electrode tip,is detected by an encoder of a servo motor for driving the moving sideelectrode tip. More particularly, the elastic displacement is determinedbased on the rotation increment of the servo motor generated from thetime when the moving side electrode tip begins to contact the fixed sideelectrode tip to when the electric current suddenly increases when themoving side electrode tip is further driven to press the fixed sideelectrode tip. A real pressing force between the electrode tips iscalculated based on the measured elastic displacement of the fixed sideelectrode tip. Then, a setting pressing force between the electrode tipsis modified to be equal to the calculated real pressing force.

[0005] However, there are the following problems with theabove-described conventional method for controlling a welding gun.

[0006] First, since a moving side portion of the welding gun includingthe moving side electrode tip and the servo motor for driving the movingside electrode tip is provided with a speed reducer, a mechanicalimpedance of the moving side portion is greater and dynamically harderthan that of a fixed side portion of the welding gun. In this instance,the mechanical impedance is defined as an impedance expressed by avector of |m, c, k|, when a movement of the electrode tip is expressedby an equation:

m·d ² x/dt ² +c·dx/dt+kx=F (t).

[0007] In a case where the vector only includes k, the mechanicalimpedance is a spring constant. The encoder is located on the oppositeside of the moving side electrode tip with respect to the speed reducerof the servo motor, so that a pressing quantity of the moving sideelectrode tip, transmission of a change in the pressing displacement andthe pressing force to the encoder through the speed reducer is a smallamount and is delayed. As a result, the responsibility is lowered, sothat it is difficult to obtain an accurate response with theconventional method, in which the welding gun is controlled based on anoutput of the encoder.

[0008] Second, since the rigidity of an arm supporting the fixed sideelectrode tip is increased so that the mechanical impedance of the fixedside portion including the fixed side electrode tip and the arm isnearly equal to the mechanical impedance of the moving side portion, thesize and the weight of the arm is large, which causes the welding gun tobe increased both in size and weight.

SUMMARY OF THE INVENTION

[0009] An object of the present invention is to provide a welding gunwhich enables control with a high response and accuracy, and to providevarious kinds of control methods conducted using the welding gun.

[0010] Another object of the present invention is to provide a weldinggun which allows an arm supporting a fixed side welding tip to bedecreased in size and rigidity, and to provide various kinds of controlmethods conducted using the welding gun.

[0011] The present invention for achieving the above objects are, asfollows:

[0012] A welding gun includes a moving side portion including a movingside welding tip and a driving device for driving the moving sideelectrode tip, and a fixed side portion including a fixed side weldingtip and an arm supporting the fixed side welding tip. A fixed sidesensor for detecting at least one of a position of the fixed sidewelding tip and a pressing force imposed on the fixed side welding tipis provided in the fixed side portion.

[0013] In the welding gun, a mechanical impedance of the fixed sideportion is smaller than that of the moving side portion. The mechanicalimpedance of the fixed side portion is set in a range where the fixedside sensor can effectively detect the at least one of the position ofthe fixed side welding tip and the pressing force imposed on the fixedside welding tip.

[0014] The fixed side sensor is any one of a force sensor, an opticaldistance sensor and a sensor using an optical fiber.

[0015] A moving side sensor for detecting at least one of a position ofthe moving side welding tip and a pressing force caused in the movingside welding tip may be provided in the moving side portion. In thatcase, the fixed side sensor and the moving side sensor constitute aredundant sensor measurement system.

[0016] A first method, which is a method of calibrating a sensorconducted using the welding gun, includes: (a) releasing the moving sidewelding tip from a position of contact with the fixed side welding tip;and (b) calibrating a reference point of at least one of a pressingforce information and a positional information of the fixed side sensor.

[0017] A second method, which is a method of calibrating a sensorconducted using the welding gun, includes: (a) increasing pressure ofthe moving side welding tip against the fixed side welding tip andplotting a pressing force and/or a positional information to obtaincharacteristic curves of the moving side sensor and the fixed sidesensor using a method of least squares; and (b) determining gains of thesensors such that the gains of the moving side sensor and the fixed sidesensor are equal to each other.

[0018] A third method, which is a control method of welding a workpiececonducted using the welding gun, includes: (a) determining whether anexpansion quantity of a welding portion of the workpiece detected by thefixed side sensor is equal to or greater than a predetermined value, andending welding of the current welding point when the expansion quantityis equal to or greater than the predetermined value; (b) increasing thewelding electric current when the expansion quantity is smaller than thepredetermined value; (c) counting the number of times the weldingelectric current is increased and determining whether a re-weldingprogram should be conducted when the counted number of times exceeds apredetermined number, and ending welding at the current welding pointwhen it is determined that the re-welding program should not beconducted; (d) conducting re-welding when it is determined that there-welding program should be conducted; and (e) determining, whenre-welding is conducted, whether the expansion quantity of the weldingportion of the workpiece during re-welding is equal to or greater thanthe predetermined value, ending welding of the current welding pointwhen the expansion quantity is equal to or greater than thepredetermined value, while issuing a warning when the expansion quantitydoes not reach the predetermined value.

[0019] A fourth method, which is a control method of welding a workpiececonducted using the welding gun, includes: (a) obtaining an expansionquantity of a welding portion of the workpiece, a position and apressing force of the fixed side welding tip, from detected valuesdetected at every moment by the fixed side sensor; (b) determiningwhether an expulsion is generated in the welding portion by comparing atleast one of a value of the pressing force and the position of the fixedside welding tip at a point when the expansion quantity begins todecrease with at least one of a value of the pressing force and theposition of the fixed side welding tip after a predetermined period oftime has passed from the beginning of the decrease in the expansionquantity; and (c) setting a welding electric current of a correspondingwelding point in a next cycle to be equal to or greater than a weldingelectric current of the current welding point when expulsion is notgenerated, while setting the welding electric current of thecorresponding welding point in the next cycle to be smaller than thewelding electric current of the current welding point when the expulsionis generated, thereby reflecting the data of the current cycle on awelding condition of the next cycle.

[0020] A fifth method, which is a method of managing a welding qualityconducted using the welding gun, includes storing data about anexpansion quantity, information about whether re-welding has beenconducted and information about whether expulsion has been generatedinto a memory at each welding point after welding of the each weldingpoint has been conducted, and periodically storing the data into amanaging system of a higher level.

[0021] A sixth method, which is a control method of welding a workpiececonducted using the welding gun, includes: (a) obtaining an expansionquantity of a welding portion of the workpiece, a position of the fixedside welding tip, a differential value of the position, a pressingforce, and a differential value of the pressing force which change atevery moment from detected values detected at every moment by the fixedside sensor; (b) determining whether a sign of an expulsion generationexists in the welding portion by comparing at least one of thedifferential value of the position of the fixed side welding tip and thedifferential value of the pressing force from the beginning of adecrease in the expansion quantity with a predetermined value, at everymoment; and (c) decreasing or stopping the welding electric current,and/or, reducing the pressing force when the sign of the expulsiongeneration exists, thereby reflecting on the welding electric currentand/or the pressing force in realtime.

[0022] A seventh method, which is a control method of welding aworkpiece conducted using the welding gun, includes: (a) determiningwhether an expansion quantity of a welding portion of the workpiecedetected by the fixed side sensor is equal to or greater than apredetermined value, and ending welding of the current welding pointwhen the expansion quantity is determined to be equal to or greater thanthe predetermined value; (b) increasing a welding electric current whenthe expansion quantity is smaller than the predetermined value; and (c)decreasing the welding electric current when a decrease in the expansionquantity is found during welding.

[0023] An eighth method, which is a control method of a pressing forceof welding conducted using the welding gun, includes: (a) detectingcontacting positions x₁ and x₁′ of the moving side welding tip and thefixed side welding tip, respectively, with a workpiece when the movingside welding tip and the fixed welding tip begin to contact theworkpiece; (b) calculating differentials between objective positionsx_(T) and x_(T)′ of the moving side welding tip and the fixed sidewelding tip, respectively, which are previously stored in a weldingrobot, and the contacting positions x₁ and x₁′, respectively; and (c)continuing pressing the workpiece, when differentials exist between theobjective positions x_(T) and x_(T)′ and the contacting positions x₁ andx₁′, respectively, based on gains proportional to the differentialsuntil the differentials become zero.

[0024] A ninth method, which is a control method of a pressing force ofwelding conducted using the welding gun, includes: (a) detectingcontacting positions x₁ and x₁′ of the moving side welding tip and thefixed side welding tip with a workpiece, respectively, when the movingside welding tip and the fixed welding tip begin to contact theworkpiece; (b) calculating differentials between objective positionsx_(T) and x_(T)′ of the moving side welding tip and the fixed sidewelding tip, respectively, which are previously stored in the weldingrobot, and the contacting positions x₁ and x₁′, respectively; (c)continuing pressing the workpiece, when differentials exist between theobjective positions x_(T) and x_(T)′ and the contacting positions x₁ andx₁′, respectively, based on gains proportional to the differentials andreturning to the step of calculating the differentials, while obtainingarriving positions x₂ and x₂′ of the moving side welding tip and thefixed side welding tip, respectively, from the current detectedpositions of the moving side welding tip and the fixed side welding tipwhen differentials do not exist; (d) calculating a pressing force P₀required for the moving side welding tip and the fixed side welding tipto reach the arriving positions; (e) adding a pressing force P₁necessary for welding to the pressing force P₀ and imposing the totalpressing force P_(T) which is a summation of the P₀ and P₁ on theworkpiece; and (f) pressing a welding electric current between themoving side welding tip and the fixed side welding tip therebyconducting welding.

[0025] A tenth method, which is a method of correcting a track of awelding robot conducted using the welding gun, includes: (a) detectingcontacting positions x₁ and x₁′ of the moving side welding tip and thefixed side welding tip with a workpiece, respectively, when the movingside welding tip and the fixed welding tip begin to contact theworkpiece; (b) calculating differentials between objective positionsx_(T) and x_(T)′ of the moving side welding tip and the fixed sidewelding tip, respectively, which are previously stored in the weldingrobot, and the contact positions x₁ and x₁′, respectively; (c)continuing pressing the workpiece, when differentials exist between theobjective positions x_(T) and x_(T)′ and the contact positions x₁ andx₁′, respectively, based on gains proportional to the differentials andreturning to the step of calculating the differentials, while obtainingarriving positions x₂ and x₂′ of the moving side welding tip and thefixed side welding tip, respectively, from current detected positions ofthe moving side welding tip and the fixed side welding tip, whendifferentials do not exist; and (d) correcting the objective positionsso that the differentials between the objective positions x_(T) andx_(T)′ and the arrival positions x₂ and x₂′, respectively, become zero.

[0026] An eleventh method, which is a method of managing a change in apositional accuracy of a welding point conducted using the welding gun,includes: (a) entering positional information x₁, x₁′, x₂ and x₂′ from adatabase storing contacting positions x₁ and x₁′ of the moving sidewelding tip and the fixed side welding tip with a workpiece at a timewhen the moving side welding tip and the fixed side welding tip begin tocontact the workpiece, and arriving positions x₂ and x₂′ of the movingside welding tip and the fixed side welding tip at a time when themoving side welding tip and the fixed side welding tip have pressed theworkpiece; (b) calculating a positional accuracy vector of a stationwith respect to a plurality of stations each having at least one robot,the matrix being defined by the following:

|Φ_(n)|=[|P₁|, |P₂|, . . . , |P_(m)|]

[0027] wherein,

[0028] n: a station number of the current station

[0029] m: the number of robots equal to or greater than 1, of station n

[0030] |P_(j)|: a positional accuracy matrix of a robot (No. j robot),obtained from the welding points P₁, P₂, . . . , and P_(k) and positionsx₁, x₁′, x₂, and x₂′ of the robot;

[0031] and (c) managing a change in a positional accuracy of the weldingpoint of the workpiece based on a value and/or values: |Φ_(n)|−|Φ_(n−1)|and/or |Φ_(n)|−|Φ₁|.

[0032] With the above welding gun, since the sensor is provided in thefixed side portion, the sensor can be disposed in a position where themechanical impedance is smaller than that of the moving side portion andwhere is not disposed via a gear such as a speed reducer from thewelding tip. As a result, a displacement of the welding tip and apressing force imposed on the welding tip can be detected with highaccuracy and a good response. By controlling the welding gun accordingto the output of the sensor, a scope of objects capable of beingcontrolled is widened and the welding gun can be made intelligent.

[0033] With the above welding gun, since the mechanical impedance of thefixed side portion is set in a range where the sensor can effectivelydetect a displacement of the fixed side welding tip and the pressingforce, the mechanical impedance of the fixed side portion can remainsmall, unlike a conventional welding gun in which the mechanicalimpedance of the fixed side portion is increased to be nearly equal to amechanical impedance of the moving side portion. As a result, the armsupporting the fixed side welding tip can be decreased both in rigidityand in size as compared with the conventional welding gun, therebymaking the welding gun compact and lightweight. Since the fixed sideportion is not provided with a speed reducer gear or the like, themechanical impedance of the fixed side portion is necessarily smallerthan that of the moving side portion. By utilizing the small mechanicalimpedance as it is, the displacement of the fixed side welding tip whenpressed is made large. Thus, sensitive and accurate detection is madepossible, which also makes the welding gun intelligent.

[0034] With the above welding gun, since the fixed side sensor is anyone of a force sensor (a load sensor), a distance sensor (a displacementsensor) and a sensor using an optical fiber, a commercial sensor can beused.

[0035] In the case where sensors are provided both in the fixed sideportion and the moving side portion of the welding gun, the fixed sidesensor and the moving side sensor can constitute a redundant sensormeasurement system. Therefore, by using one sensor, calibration of areference point, confirmation of a normal operation, etc. of the othersensor can be performed.

[0036] In the first method that is a method of calibrating a sensorconducted using the welding gun, since the reference point of the fixedside sensor is calibrated in a state that the moving side welding tip isreleased from the fixed side welding tip, the reference point of thefixed side sensor can be calibrated based on an output from the movingside sensor.

[0037] In the second method that is a method of calibrating a sensorconducted using the welding gun, since the moving side welding tip ispressed against the fixed side welding tip and the gains of the movingside sensor and the fixed side sensor are adjusted, one sensor cancalibrate the other sensor.

[0038] In the third method that is a method of controlling weldingconducted using the welding gun, the method can be conducted withoutusing the moving side sensor. Also, the method can be conducted even ifthe driving device for the moving side portion is not a servo motor butan air cylinder. Further, a re-welding feedback control can beperformed. When the expansion quantity of the welding portion duringwelding does not reach the predetermined value even though the weldingelectric current is increased which may be caused by dust or the likeadhering to the workpiece or by a malfunction of the apparatusre-welding is conducted, because it may work in the case of the adhesivedust or the like. If the expansion quantity of the welding portion stilldoes not reach the predetermined value despite re-welding, it isdetermined that the apparatus has malfunctioned, and a warning isissued. The warning may be substituted with stoppage.

[0039] In the fourth method that is a method of controlling weldingconducted using the welding gun, the method can be conducted withoutusing the moving side sensor. Also, the method can be conducted even ifthe driving device for the moving side portion is not a servo motor butan air cylinder. Further, generation of the expulsion in thecorresponding welding point in the next cycle can be suppressed. In anormal condition, the expansion quantity (or the pressing force) of thewelding portion during welding generally increases and forms a positiveexponential curve (e^(t)) and decreases and forms a negative exponentialcurve (e^(−t)) after reaching a peak (where the welding electric currentstops). In contrast, when the expulsion is generated, the expansionquantity (or the pressing force) suddenly decreases simultaneously withthe expulsion generation and then returns to a value smaller than thatat the beginning of welding. When the expansion quantity is continuouslydetected and it is detected that the expansion quantity suddenlydecreases as compared with the predetermined curve of the normalcondition, it can be determined that the expulsion has just beengenerated, and the welding electric current of the corresponding weldingpoint in the next cycle should be decreased. As a result, generation ofthe expulsion in the corresponding welding point in the next cycle canbe suppressed. With respect to control of the welding gun, since a verysmall change in the expansion quantity has to be detected, it isdifficult in the detected quantity and responsibility to perform anaccurate control of the expansion quantity by using the conventionalservo motor encoder. By using a welding gun provided with the fixed sidesensor which can sense data on the order of every 10⁻⁶ seconds, anaccurate detection and the aforementioned control are possible.

[0040] In the fifth method that is a method of managing a weldingquality conducted using the welding gun, since data about the expansionquantity, information about whether re-welding has been performed andinformation about whether expulsion has been generated are stored intothe memory at every welding point in the third and the fourth methods,and are periodically stored into the managing system of a higher level,welding quality can be managed.

[0041] In the sixth method that is a method of controlling weldingconducted using the welding gun, the method can be conducted withoutusing the moving side sensor. Also, the method can be conducted even ifthe driving device for the moving side portion is not a servo motor butan air cylinder. Further, generation of the expulsion in the currentwelding point can be suppressed. In a normal condition, the expansionquantity (or, the pressing force) of the welding portion during weldinggenerally increases and forms a positive exponential curve (e^(t)) anddecreases and forms a negative exponential curve (e^(−t)) after reachingthe peak (where the welding electric current stops). In contrast, whenthe expulsion is generated, a gradient of the curve of the expansionquantity (or the pressing force) begins to decrease, then rapidlydecreases and returns to a value smaller than that at the beginning ofwelding. When the expulsion is detected at every moment during weldingand the gradient of the curve is then calculated at every moment and itis found that the gradient decreases more greatly than a predeterminedallowable value, it is determined that a sign of the expulsiongeneration exists and the welding electric current is controlled to bedecreased. As a result, generation of an expulsion in the currentwelding point can be suppressed. In the control, since a very smallchange in the gradient has to be detected, it is difficult in quantityand responsibility to perform an accurate control of the expansionquantity by using the conventional servo motor encoder. By using thewelding gun provided with the fixed side sensor which can perform acontrol of the order of 10⁻⁶ seconds, an accurate detection and theaforementioned control are possible.

[0042] In the seventh method that is a method of controlling weldingconducted using the welding gun, the method can be conducted withoutusing the moving side sensor. Also, the method can also be conductedeven if the driving device for the moving side portion is not a servomotor but an air cylinder. Further, in the seventh control method, thewelding strength can be controlled. In general, there is a correlationbetween a welding electric current, and a nugget size and a thermalexpansion quantity. In addition, there is a correlation between thenugget size and the thermal expansion quantity, and a welding strength.It is generally considered that the spot welding has a sufficientwelding strength when the thermal expansion quantity reaches thepredetermined value. Therefore, the welding electric current isincreased until the thermal expansion quantity reaches the predeterminedvalue. However, when the thermal expansion quantity decreases duringpressing, which means that some welding expulsion is generated, thewelding electric current is decreased. By repeating this routine duringwelding, spot welding having a necessary welding strength can beconducted in a minimum time period under a condition that expulsion isnot generated.

[0043] In the eighth method that is a method of controlling weldingconducted using the welding gun, both the fixed side sensor and themoving side sensor are used. The driving device for the moving sideportion may be a servo motor or an air cylinder. In the eighth method,the control for the pressing force is conducted based on informationabout positions x₁, x₁′ of the moving side welding tip and the fixedside welding tip at a time when the welding tips reach the contactingpoints with a workpiece (at that time the electric current of the motorfor diving the moving side welding tip suddenly increases). Moreparticularly, differentials between the objective positions x_(T) andx_(T)′ of the moving side welding tip and the fixed side welding tip andthe contacting positions x₁ and x₁′ are calculated, respectively. Whensome differentials exist, which means that the workpieces have aclearance therebetween, the pressing force continues to be imposed onthe workpiece based on gains proportional to the differentials. Due tothis pressing, the workpieces are pressed and the clearance between theworkpieces is eliminated. When the differentials become zero or thedifferentials do not exist, the arriving positions x₂ and x₂′ of themoving side welding tip and the fixed side welding tip are obtained fromthe currently detected positions x₁ and x₁′ of the moving side weldingtip and the fixed side welding tip. Due to this operation, even if aclearance exists between the workpieces, the clearance is eliminated andthen welding is conducted, so that a spot welding of a high qualitycausing no separation can be performed.

[0044] In the ninth method that is a method of controlling a pressingforce of welding conducted using the welding gun, both the fixed sidesensor and the moving side sensor are used. The driving device for themoving side portion may be a servo motor or may be an air cylinder. Inthe ninth method, controlling the pressing force is conducted based oninformation about positions x₁ and x₁′ of the moving side welding tipand the fixed side welding tip when the welding tips reach thecontacting points with a workpiece (at that time the electric current ofthe motor for driving the moving side welding tip suddenly increases).More particularly, the differentials between the objective positionsx_(T) and x_(T)′ of the moving side welding tip and the fixed sidewelding tip and the contacting positions x₁ and x₁′ are calculated,respectively. When differentials exist, which means that the workpieceshave a clearance therebetween, the pressing force continues to beimposed on the workpiece based on gains proportional to thedifferentials. Due to this pressing, the workpieces are pressed and theclearance between the workpieces is eliminated. When the differentialsbecomes zero or the differentials do not exist, the arriving positionsx₂ and x₂′ of the moving side welding tip and the fixed side welding tipare obtained from the currently detected positions x₁ and x₁′ of themoving side welding tip and the fixed side welding tip. Then, thepressing force P₀ required for the welding tips to reach the respectivearriving points is calculated. The pressing force P₀ is a force imposedon the workpieces in order to eliminate the clearance between theworkpieces and therefore is a spring back force of the workpieces. Apressing force P₁ necessary for welding is further added to the pressingforce P₀, and the total pressing force P₀ of P₁ and P_(T) is imposed onthe workpieces. Due to this operation, even if a clearance existsbetween the workpieces, the pressing force P₁ necessary for welding canbe imposed on the workpieces, so that a spot welding of a high qualityhaving a sufficient pressing force can be performed.

[0045] In the tenth method that is a method of correcting a track of awelding robot conducted using the welding gun, both the fixed sidesensor and the moving side sensor are used. The driving device for themoving side portion may be a servo motor or may be an air cylinder. Inthe tenth method, controlling the pressing force is conducted based oninformation about positions x₁ and x₁′ of the moving side welding tipand the fixed side welding tip when the welding tips reach thecontacting points with a workpiece (at that time the electrical currentof the motor for driving the moving side welding tip suddenlyincreases). More particularly, the differentials between the objectivepositions x_(T) and x_(T)′ of the moving side welding tip and the fixedside welding tip and the contacting positions x₁ and x₁′ are calculated,respectively. When differentials exist, which means that the workpieceshave a clearance therebetween, the pressing force continues to beimposed on the workpieces based on gains proportional to thedifferential. Due to this pressing, the workpieces are pressed and theclearance between the workpieces is eliminated. When the differentialsbecome zero or the differentials do not exist, the arriving positions x₂and x₂′ of the moving side welding tip and the fixed side welding tipare obtained from the currently detected positions x₁ and x₁′ of themoving side welding tip and the fixed side welding tip. The arrivingpositions x₂ and x₂′ are real arriving positions. By correcting theobjective positions x_(T) and x_(T)′ to the real positions x₂ and x₂′,the track of the welding robot is modified and is prepared for weldingof the corresponding welding point of the next cycle.

[0046] In the eleventh method that is a method of managing a change in apositional accuracy of a welding point conducted using the welding gun,both the fixed side sensor and the moving side sensor are used. Thedriving device for the moving side portion may be a servo motor or maybe an air cylinder. In the eleventh method, the positional accuracymatrix of each station |Φ_(n)| is calculated from the contactingpositions and pressing positions at the respective welding points andthen the change in the positional accuracy at the welding point of theworkpiece is managed based on the value and/or values of|Φ_(n)|−|Φ_(n−1)| and/or |Φ_(n)|−|Φ₁|. Therefore, a deformation of theworkpiece due to welding can be suppressed, for example, by changing thewelding order of the welding points thereby obtaining an optimum weldingorder in which that the value of |Φ_(n)|−|Φ_(n−1)| is further decreased.

BRIEF DESCRIPTION OF THE DRAWINGS

[0047] The objects, features, and advantages of the present inventionwill become more apparent and will more readily be appreciated by thefollowing detailed description of the preferred embodiments illustratedby the accompanying drawings, in which:

[0048]FIG. 1 is a side view of a welding gun according to an embodimentof the present invention;

[0049]FIG. 2a is a partial cross-sectional view of the welding gun inaccordance with the present invention;

[0050]FIG. 2b is a perspective view of an example of the fixed sidesensor shown in FIG. 2a.

[0051]FIG. 3a is a cross-sectional view of the welding gun according tothe embodiment of the present invention;

[0052]FIG. 3b is an enlarge view of an example of the fixed side sensorshown in FIG. 3a.

[0053]FIG. 4 is a cross-sectional view of another example of a fixedside sensor and the vicinity, of the welding gun according to theembodiment of the present invention;

[0054]FIG. 5 is a system diagram illustrating the welding gun accordingto the embodiment of the present invention, and a welding robot to whichthe welding gun is coupled;

[0055]FIG. 6 is a flowchart illustrating a control routine of a methodfor calibrating a sensor, which is conducted using the welding gunaccording to the embodiment of the present invention;

[0056]FIG. 7 is a graph showing plots of a sensor calibration and linesdrawn by a method of least square;

[0057]FIG. 8 is a flowchart illustrating a routine for an operationprogram conducted after the routine of FIG. 6;

[0058]FIG. 9 is a flowchart illustrating a routine for re-weldingfeedback control, which is conducted using the welding gun according tothe embodiment of the present invention;

[0059]FIG. 10 is a cross-sectional view illustrating the relationshipbetween a welding tip and a workpiece during the routine of FIG. 9;

[0060]FIG. 11 is a flowchart illustrating a routine for controllingexpulsion generation, which is conducted using the welding gun accordingto the embodiment of the present invention;

[0061]FIG. 12 is a flowchart illustrating a routine for controlling anexpulsion generation in the current welding point, which is conductedusing the welding gun according to the embodiment of the presentinvention;

[0062]FIG. 13 is a graph illustrating a relationship between theexpulsion generation, and a pressing force or a position of the weldingtip;

[0063]FIG. 14 is a flowchart illustrating a routine for controllingwelding strength, which is conducted using the welding gun according tothe embodiment of the present invention;

[0064]FIG. 15 is a cross-sectional view illustrating a relationshipbetween workpieces having a clearance therebetween and positions of thewelding tips;

[0065]FIG. 16 is a flowchart illustrating a routine including aclearance reduction control, a pressing force control and a robot trackcontrol, which is conducted using the welding gun according to theembodiment of the present invention;

[0066]FIG. 17 is a schematic system diagram illustrating an example ofan objective to which a managing method of a change in a positionalaccuracy at a welding point is applied using the welding gun accordingto the embodiment of the present invention;

[0067]FIG. 18 shows a positional accuracy matrix of the robot used inthe managing method of a change in a positional accuracy at a weldingpoint, which is conducted using the welding gun according to theembodiment of the present invention;

[0068]FIG. 19 is a plan view of stations each having welding robots,which is an example of the objective to which the managing method of achange in a positional accuracy at a welding point is applied using thewelding gun according to the embodiment of the present invention;

[0069]FIG. 20 is a flowchart illustrating a routine of managing a changein positional accuracy at a welding point, which is conducted using thewelding gun according to the embodiment of the present invention; and

[0070]FIG. 21 is a flowchart illustrating a routine in a case wherecontrolling a workpiece deformation to minimum is conducted using themanaging method of a change in a positional accuracy at a welding point,which is conducted using the welding gun according to the embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0071] FIGS. 1-5 illustrate a welding gun according to an embodiment ofthe present invention. FIGS. 6-21 illustrate various kinds of methodsconducted using the welding gun according to the embodiment of thepresent invention. The methods include, for example, a calibrationmethod of a sensor, a confirmation method of a sensor operation, acontrol method of welding, and a managing method of welding data.

[0072] First, the welding gun according to the embodiment of the presentinvention will be explained with reference to FIGS. 1-5.

[0073] As illustrated in FIG. 1, the welding gun 1 includes a movingside portion 10 including a moving side welding tip 11 and a drivingdevice 12 for driving the moving side welding tip 11, and a fixed sideportion 20 including a fixed side welding tip 21 and an arm 22supporting the fixed side welding tip 21.

[0074] A fixed side sensor 23 is disposed in the fixed side portion 20for detecting at least one of a position of the fixed side welding tip21 and a pressing force imposed on the fixed side welding tip 21.

[0075] The welding gun 1 is supported by a welding robot, for example,of a six-axis, and a movement and a pressing force of the welding gun 1is controlled by a control unit 30. The control unit 30 is a computer,for which a control unit of the welding robot 40 may be used.

[0076] Preferably, a moving side sensor 13 is also disposed in themoving side portion 10 for detecting at least one of a position of themoving side welding tip 11 and a pressing force caused in the movingside welding tip 11.

[0077] The driving device 12 may be a servo motor or an air cylinder.When the driving device 12 is a servo motor, a speed of the servo motoris reduced by a speed reducer 17 (which is generally a speed reducer ofa motor with speed reducer). The rotation of the driving device isconverted into a reciprocating linear motion by a mechanism ofconverting a rotation to a linear motion, such as a ball screw 16 or thelike, and then is transmitted to a moving side welding tip 11.

[0078] When the driving device 12 is a servo motor, the moving sidesensor 13 includes an encoder (a rotational position detection sensor)coupled to the servo motor. Reference numeral 18 expresses a transformerfor a welding electric current.

[0079] The fixed side sensor 23 is disposed in the arm 22. The arm 22may have a substantially C-shaped configuration, or may have otherconfigurations, for example, a flat plate. The fixed side sensor 23 isdisposed in a portion of the arm 22 which generates a compression stressor a tensile stress due to a bending moment caused when the pressingforce is imposed on the fixed side welding tip 21.

[0080] The fixed side sensor 23 is constructed of, for example, a loadsensor (a force sensor) 23A, an optical distance sensor (a positionsensor) 23B, or a sensor 23D using an optical fiber. Informationdetected by the fixed side sensor 23 is transmitted in the form of anelectrical signal.

[0081]FIGS. 2a-2 b illustrate an example where the fixed side sensor 23is the force sensor 23A and is disposed in a portion of the arm 22, moreparticularly, in a lower-half portion with respect to a center line ofthe arm, which generates the compression stress due to a bending momentcaused when a welding pressing force is imposed on the fixed sidewelding tip 21.

[0082]FIGS. 3a-3 b illustrate an example where the fixed side sensor 23is the optical distance sensor 23B and is disposed in a portion the arm22, more particularly, in an upper half portion with respect to thecenter line of the arm (or in the lower-half portion in a case ofcompression stress), which generates a tensile stress (or compressionstress in the case of the lower-half portion) due to a bending momentcaused when a welding pressing force is imposed on the fixed sidewelding tip 21. Based on a dislocation between a light issue point and alight return point detected when the light is transmitted from theoptical distance sensor 23B is reflected at a reflecting plate 23C andreturns to the sensor, the magnitude of the bending moment and themagnitude of the pressing force imposed on the fixed side welding tip 21are detected.

[0083]FIG. 4 illustrates an example where the fixed side sensor 23 is anoptical fiber 23D. The optical fiber is provided both in a portion ofthe arm 22 which generates tensile stress due to the bending momentcaused when the pressing force is imposed on the fixed side welding tip21 and in a portion of the arm which generates compression stress due tothe bending moment caused when the pressing force is applied. When thepressing force is imposed on the fixed side welding tip 21, an opticalpath length difference generated in the optical fibers is detected by adetector 23E, so that the magnitude of the pressing force caused on thefixed side welding tip 21 is detected.

[0084]FIG. 5 is a system diagram illustrating the device of FIG. 1 inthe form of a system. The output of the moving side sensor 13 isamplified by an amplifier 14. In a case of an analog signal, the analogsignal is converted into a digital signal by an AID (analog/digital)converter and then fed to the control unit 30. In the same manner, theoutput of the servo motor 12 is amplified by an amplifier 15. In a caseof an analog signal, the analog signal is converted into a digitalsignal by the A/D converter and then fed to the control unit 30. In thesame manner, the output of the fixed side sensor 23 is amplified by anamplifier 24. In a case of an analog signal, the analog signal isconverted into a digital signal by, the A/D converter and then fed tothe control unit 30.

[0085] In FIG. 5, the mechanical impedance of the moving side portion 10is expressed by reference G₁ and the mechanical impedance of the fixedside portion 20 is expressed by reference G₂. In a conventionalstructure, since G₁ is generally greater than G₂ because a gear or thelike are provided in the moving side portion 10, G₂ is designed to beapproximately equal to G₁. In the embodiment of the present invention,the mechanical impedance G₂ is permitted to be smaller than G₁, and G₂remains to be smaller than G₁.

[0086] The relationship between G₁ and G₂ is selected to be, forexample, G₂≦(1/2) G₁. Preferably, the relationship is G₂≦(1/5) G₁. Morepreferably, the relationship is G₂≦(1/10) G₁. Due to the relationship,the mechanical impedance G₂ of the fixed side portion 20 is set in arange where the position of the fixed side welding tip 21 and thepressing force imposed on the fixed side welding tip 21 can beeffectively detected by the fixed side sensor 23.

[0087] More particularly, when the moving side welding tip 11 is drivento press the fixed side welding tip 21 thereby causing displacements inthe moving side welding tip and the fixed side welding tip 21,deformation caused in the fixed side portion 20 is greater than thatcaused in the moving side portion 10, that is, in a case satisfying therelationship: G₂≦(1/10) G₁, the deformation in the fixed side portion 20occupies more than 90% of the total amount of deformation. As a result,the output of the fixed side sensor 23 is greater than that of themoving side sensor 13, so that controlling the welding gun based on theoutput of the fixed side sensor 23 is easier and more accurate.

[0088] With the above-described structure, it becomes possible toimplement various types of controls not possible with the conventionalstructure having a moving side sensor 13 only. Accordingly, the weldinggun 1 is made intelligent.

[0089] In a case where both the moving side sensor 13 and the fixed sidesensor 23 are provided, the sensors 13 and 14 constitute a redundantsensor measurement system. The redundant sensor measurement system isdefined as a system including more than two sensors, any one of whichcan perform calibration and operational confirmation, etc. of the othersensor.

[0090] Operations and advantages of the aforementioned welding gun 1 areas follows:

[0091] First, since the fixed side sensor 23 is disposed in the fixedside portion 20, the sensor 23 can be disposed in a portion which has asmaller mechanical impedance than the moving side portion 10 and neednot be spaced via a gear such as a speed reducer from the welding tip,so that displacement of the welding tip and the pressing force can bedetected with a high accuracy and a good response. By controlling thewelding gun 1 based on the detected values, the scope of objects capableof being controlled is widened.

[0092] Second, since the magnitude of the mechanical impedance G₂ of thefixed side portion 20 is set in a range where the displacement of thefixed side welding tip 21 and the pressing force imposed on the fixedside welding tip 21 can be effectively detected by the fixed side sensor23, the mechanical impedance G₂ is permitted to remain to be small,unlike the conventional structure in which the mechanical impedance G₂of the fixed side portion 20 is made to be nearly equal to themechanical impedance G₁ of the moving side portion. As a result, therigidity and the size of the arm 22 supporting the fixed side weldingtip 21 can be smaller than those of the conventional structure, therebyenabling the welding gun 1 to be compact and lightweight. Since thefixed side portion 20 is not provided with a gear reducer, themechanical impedance G₂ is smaller than that of the moving side portion.As a result of this small mechanical impedance, the displacement of thefixed side welding tip 21 when pressed is large, so that sensitive andaccurate detection can be conducted.

[0093] Third, in the case where the sensors are disposed both in thefixed side portion 20 and the moving side portion 10, respectively, thefixed side sensor 23 and the moving side sensor 13 constitute aredundant sensor measurement system. In the system, one sensor canperform calibration of reference points and confirmation of normaloperation, etc. of the other sensor.

[0094] Next, various kinds of methods conducted using the welding gun 1will be explained.

[0095] A first method, which is a method of calibrating a referencepoint of a sensor, and a second method, which is a method of calibratinga pressing force characteristic, will be explained with reference toFIGS. 6, 7 and 8.

[0096] The first method of calibrating the sensors 13 and 23 isconducted using the welding gun 1. The welding gun 1 includes a movingside portion 10 including a moving side welding tip 11 and a drivingdevice 12 for driving the moving side welding tip 11, and a fixed sideportion 20 including a fixed side welding tip 21 and an arm 22supporting the fixed side welding tip 21. A fixed side sensor 23 isdisposed in the fixed side portion 20 for detecting at least one of aposition of the fixed side welding tip 21 and a pressing force imposedon the fixed side welding tip 21. A moving side sensor 13 is disposed inthe moving side portion 10 for detecting a position of the moving sidewelding tip 11 and a pressing force caused in the moving side weldingtip 11.

[0097] The first method of calibrating the reference points of thesensors 13 and 23 includes steps of: releasing the moving side weldingtip 11 from the fixed side welding tip 21; and calibrating the referencepoints of a pressing force and/or a positional information of the fixedside sensor 23 based on the output of the moving side sensor 13 and theelectric current output of the servo motor 12.

[0098] The second method of calibrating the pressing forcecharacteristics of the sensors 13 and 23 includes: increasing pressingof the moving side welding tip 11 against the fixed side welding tip 21after having calibrated the reference points of the sensors 13 and 23,and plotting a pressing force information and/or a positionalinformation to obtain characteristic curves of the moving side sensor 13and the fixed side sensor 23 using a method of least square; anddetermining gains of the moving side sensor 13 and the fixed side sensor23 so that the gains of the moving side sensor 13 and the fixed sidesensor 23 are coincident with each other.

[0099]FIG. 6 illustrates the calibration routine. The routine of FIG. 6is stored in the control unit 30. The calibration routine includes aroutine for calibrating the reference point and a routine forcalibrating the pressing force.

[0100] The calibration routine is entered at an appropriate timing, forexample, at a time before and after spot-welding of an automobile isconducted. However, entering the calibration routine is not limited tothe above timing, and the calibration routine may be entered duringperforming the welding operation program. At step 101, signals issuedfrom the servo motor 12, the moving side sensor 13 and the fixed sidesensor 23 are amplified and input to the control unit 30. Then, themoving side welding tip 11 is released from the fixed side welding tip21, and the reference point, for example zero point, of the pressingforce information or the positional information of the fixed side sensor23, or of both the moving side sensor 13 and the fixed side sensor 23are calibrated. The release of the moving side welding tip 11 from thefixed side welding tip 21 is recognized by the electric current outputof the servo motor 12 (from which it can be judged that the moving sidewelding tip 11 is in a released condition, because the welding electriccurrent is suddenly increased when the moving side welding tip 11 beginsto be pressed against the fixed side welding tip 21), and a positionalsignal from the moving side sensor 13, for example, the encoder. Sincethe outputs of the servo motor 12 and the moving side sensor 13 (forexample, the encoder) generally have a high degree of accuracy andreliability, the reference point of the fixed side sensor 23 iscalibrated according to the reference point of the moving side sensor13.

[0101] Then, the routine proceeds to calibrating the pressing force(steps 102-107). At step 102, the moving side welding tip 11 is pressedagainst the fixed side welding tip 21 to impose the pressing forcethereon. The pressing force is increased at step 104 by a predeterminedload increment, for example, by every 100 kg, until the pressing forcereaches the maximum value at step 103. The pressing force is increasedby increasing the electric current of the servo motor 12, because alinear relationship exists between the servo motor electric current andthe pressing force. At step 105, as illustrated in FIG. 7, the pressingforce information (or the positional information which is proportionalto the pressing force) which are the outputs of the moving side sensor13 and the fixed side sensor 23 is plotted at every load, and thencharacteristic curves 53 and 54 of outputs of the moving side sensor 13and the fixed side sensor 23 are obtained using the method of leastsquares. Then, the gains of the moving side sensor 13 and the fixed sidesensor 23 are calculated so that the characteristic curves 53 and 54 arecoincident with each other. In this instance, since a workpiece is notplaced between the welding tips of the welding gun, the sensors 13 and23 do not receive a reaction force from a workpiece, thus thecharacteristic curves ought to be coincident with each other. At step106, a decision is made by an operator whether to set the calculatedgains. If the decision is made to set the gains, the gains are set atstep 107 and the calibration of the pressing force (or the position) isconducted, and the routine proceeds to the next operation programroutine illustrated in FIG. 8. When the calculated gains are not set atstep 107, the routine returns to step 105, where the gains arere-calculated. Due to the this operation, the pressing forcecharacteristic (or the positional characteristic) can be calibrated.

[0102] Then, the routine proceeds to the operation program of FIG. 8(steps 108-111). The routine is initiated at each welding point. At step108, it is determined whether pressing of the welding gun is beingconducted. If the welding electric current of the servo motor 12 isincreasing, it can be determined that pressing of the welding gun isbeing conducted. When it is determined that the pressing is conducted,various kinds of monitoring can be conducted at step 109. During themonitoring, there may occur a case where the outputs of the moving sidesensor 13 and the output of the fixed side sensor 23 are not coincidentwith each other due to a reaction force from the workpiece at thecurrent welding point. In such a case, when welding is finished at thecurrent welding point, that is, pressing of the welding gun is not beingconducted, the routine proceeds to step 110, where the reference point,for example a zero point, is reset. This reset is a reset to thereference point which has been calibrated in the routine illustrated inFIG. 6. Then, at step 111, it is determined whether or not welding at aplurality of the welding points or at all of the welding points isfinished. When the welding is not finished, the routine returns to step108 and welding at the next welding point is repeated. When the weldingis finished, the routine proceeds to an end.

[0103] A third method, a re-welding feedback control method, will beexplained with reference to FIGS. 9 and 10.

[0104] The third method is conducted using the welding gun 1. Thewelding gun 1 includes a moving side portion 10 including a moving sidewelding tip 11 and a driving device 12 for driving the moving sidewelding tip 11 and a fixed side portion 20 including a fixed sidewelding tip 21 and an arm 22 supporting the fixed side welding tip 21. Afixed side sensor 23 is disposed in the fixed side portion 20 fordetecting at least one of a position of the fixed side welding tip 21and a pressing force imposed on the fixed side welding tip 21. A movingside sensor 13 is not necessarily disposed in the moving side portion10. The driving device 12 may be a servo motor, or may be an aircylinder.

[0105] The third method includes:

[0106] {circle over (1)} determining whether or not an expansionquantity of a welding portion of the workpiece detected by the fixedside sensor 23 is equal to or greater than a predetermined value, endingwelding at the current welding point when the expansion quantity isequal to or greater than the predetermined value;

[0107] {circle over (2)} increasing the welding electric current I(t)when the expansion quantity is smaller than the predetermining value;

[0108] {circle over (3)} counting the number of times of increasing thewelding electric current I(t) and determining whether a re-weldingprogram should be conducted when the counted number of times exceeds apredetermined number, ending welding at the current point when it isdetermined that the re-welding program should not be conducted;

[0109] {circle over (4)} conducing re-welding when it is determined thatthe re-welding program should be conducted; and

[0110] {circle over (5)} determining when re-welding is conductedwhether the expansion quantity of the welding portion of the workpieceduring re-welding is equal to or greater than the predetermined value,ending welding of the current welding point when the expansion quantityis equal to or greater than the predetermined value while issuing awarning when the expansion quantity does not reach the predeterminedvalue.

[0111] In FIG. 10, since the mechanical impedance of the fixed sideportion 20 is smaller than that of the moving side portion 10, almostall of the thermal expansion quantity (a displacement) of a weldingportion 61 of a workpiece 60 is detected by the fixed side sensor 23.When the thermal expansion quantity of the welding portion 61 exceedsthe predetermined value, it can be judged that a satisfactory weldinghas been conducted. Further, there is a correlation between the weldingelectric current and the thermal expansion quantity of the weldingportion. More specifically, the more the welding electric current is,the more the thermal expansion quantity of the welding portion 61 is. Itwas found, however, that too much electric current causes dispersion ofa weld magma (an expulsion generation), leading to a sudden decrease inthe pressing force.

[0112]FIG. 9 illustrates a control routine of the re-welding feedbackcontrol method. At step 201, it is determined at every moment whether ornot the thermal expansion quantity δ of the current welding portion isequal to or greater than a predetermined value δ_(s). When the thermalexpansion quantity δ exceeds the predetermined value δ_(s), the routineproceeds to step 208 where the thermal expansion quantity is stored intoa database (D/B), and proceeds to an end step wherein the welding isfinished at the current welding point and the robot moves to the nextwelding point. When it is determined that the thermal expansion quantityδ is smaller than the predetermined value δ_(s) at step 201, the routineproceeds to step 202, where the welding electric current I(t) isincreased and then returns to step 201. Then, the number of times ofpassing through steps 201 and 202 is counted at step 203. When thenumber exceeds a predetermined number, the routine proceeds to step 204,where it is determined whether a re-welding program should be conducted.The reason why re-welding is conducted is that the re-welding programsometimes works well in a case where dust or the like adheres to theworkpiece. When it is determined, by an operator or by comparing with apredetermined condition, that the re-welding program should not beconducted, the data are stored into the database at step 208 and theroutine then proceeds to the end step, wherein welding at the currentwelding point is finished. When it is determined that the re-weldingprogram should be conducted at step 204, the routine proceeds to step205, where the re-welding is conducted. After the re-welding, theroutine proceeds to step 206, where it is determined whether the thermalexpansion quantity δ of the re-welding portion is equal to or greaterthan the predetermined value δ_(s). When the thermal expansion quantityδ exceeds the predetermined value δ_(s), the routine proceeds to step208 where the data on the thermal expansion quantity are stored into thedatabase (D/B), and then proceeds to the end step where welding at thecurrent welding point is finished. When the thermal expansion quantity δdoes not reach the predetermined value δ_(s), the routine proceeds tostep 207, where a warning is issued.

[0113] A fourth method, a control method for suppressing an expulsiongeneration in a corresponding welding point in a next cycle, and a fifthmethod, which is a method of managing a welding quality, will beexplained with reference to FIGS. 11 and 13.

[0114] The fourth and fifth methods are conducted using the welding gun1. The welding gun 1 includes a moving side portion 10 including amoving side welding tip 11 and a driving device 12 for driving themoving side welding tip 11 and a fixed side portion 20 including a fixedside welding tip 21 and an arm 22 supporting the fixed side welding tip21. A fixed side sensor 23 is disposed in the fixed side portion 20 fordetecting at least one of a position of the fixed side welding tip 21and a pressing force imposed on the fixed side welding tip 21. A movingside sensor 13 is not necessarily disposed in the moving side portion10. The driving device 12 may be a servo motor, or may be an aircylinder.

[0115] The fourth method includes:

[0116] {circle over (1)} obtaining a thermal expansion quantity of awelding portion 61, a position and a pressing force of the fixed sidewelding tip 21, which change at every moment, from detected valuesdetected at every moment by the fixed side sensor 23;

[0117] {circle over (2)} determining whether an expulsion is generatedin the welding portion 61 by comparing a value of the pressing forceand/or the position of the fixed side welding tip 21 at a beginning of adecrease in the thermal expansion quantity with a value of the pressingforce and/or the position of the fixed side welding tip 21 after apredetermined period of time has passed from the beginning of thedecrease in the thermal expansion quantity; and

[0118] {circle over (3)} setting a welding electric current I(t)′ of acorresponding welding point in a next cycle to be equal to or greaterthan a welding electric current I(t) of the current welding point whenthe expulsion is not generated, while setting the welding electriccurrent I(t)′ of the corresponding welding point in the next cycle to besmaller than the welding electric current I(t) of the current weldingpoint when the expulsion is generated, thereby reflecting the data ofthe current cycle on a welding condition of the next cycle.

[0119]FIG. 11 illustrates a control routine in accordance with thefourth method, more particularly, the control method which suppressesthe expulsion generation in the corresponding welding point in the nextcycle. The control routine is stored in the control unit 30.

[0120] In the control routine, at step 301, it is determined whether thethermal expansion quantity is equal to or greater than a predeterminedvalue (which is a thermal expansion quantity when a satisfactory weldinghas been conducted). When the thermal expansion quantity is equal to orgreater than the predetermined value, the routine proceeds to step 306,where the value of the thermal expansion quantity is stored into thedatabase, and then proceeds to an end step. When it is determined thatthe thermal expansion quantity does not reach the predetermined value,the routine proceeds to step 302, where the current welding electriccurrent I(t) is increased by a predetermined value, and then proceeds tostep 303. At step 303, it is determined whether or not the thermalexpansion quantity begins to decrease. When the thermal expansionquantity does not begin to decrease, it is determined that the expulsionis not generated, and the routine returns to step 301. When it isdetermined that the thermal expansion quantity begins to decrease atstep 303, the routine proceeds to step 304, where it is determinedwhether or not a differential ΔF (=F₁−F₂, see FIG. 13) between thepressing force (F₁) or the position of the fixed side welding tip at thebeginning of a decrease in the thermal expansion quantity and a pressingforce (F₂) or a position of the fixed side welding tip after apredetermined period of time Δt (see FIG. 13) has passed from thebeginning of a decrease in the thermal expansion quantity exceeds apredetermined value F₀. When the differential ΔF is less than thepredetermined value F₀, it is determined that the expulsion is notgenerated and the routine then returns to step 301. When thedifferential ΔF exceeds the predetermined value F₀, which means that asudden change (decrease) in the pressing force or the position of thefixed side welding tip is caused, it is determined that the expulsion isgenerated and the routine proceeds to step 305, where the weldingelectric current I(t)′ of the corresponding welding point in the nextcycle is set to be smaller than the welding electric current I(t) of thecurrent welding point.

[0121] Due to this operation, an expulsion is suppressed from generatingin the corresponding welding point in the next cycle.

[0122] Further, in accordance with the fifth method, as illustrated inFIG. 11, data about the thermal expansion quantity after the currentwelding is conducted, information about whether re-welding has beenconducted and information about whether the expulsion has been generatedare stored into a memory at each welding point at step 306. Then, thesedata are periodically stored into a managing system of a higher level.Due to this operation, managing a welding quality is possible.

[0123] A sixth method, a control method for suppressing an expulsiongeneration in the current welding point, will be explained withreference to FIGS. 12 and 13.

[0124] The sixth control method is conducted using the welding gun 1.The welding gun 1 includes a moving side portion 10 including a movingside welding tip 11 and a driving device 12 for driving the moving sidewelding tip 11 and a fixed side portion 20 including a fixed sidewelding tip 21 and an arm 22 supporting the fixed side welding tip 21. Afixed side sensor 23 is disposed in the fixed side portion 20 fordetecting at least one of a position of the fixed side welding tip 21and a pressing force imposed on the fixed side welding tip 21. A movingside sensor 13 is not necessarily disposed in the moving side portion10. The driving device 12 may be a servo motor, or may be an aircylinder.

[0125] The sixth method includes:

[0126] {circle over (1)} obtaining a thermal expansion quantity of awelding portion 61, a position of the fixed side welding tip 21, adifferential value (dx/dt) of the position, a pressing force, and adifferential value (dF/dt) of the pressing force, which change at everymoment, from detected values detected at every moment by the fixed sidesensor 23;

[0127] {circle over (2)} determining whether or not a sign of anexpulsion generation exists in the welding portion 61 by comparing thedifferential value (dx/dt) of the position of the fixed side welding tip21 and/or the differential value (dF/dt) of the pressing force from thepoint at which the thermal expansion quantity begins to decrease with apredetermined value or values, at every moment; and

[0128] {circle over (3)} decreasing or stopping the welding electriccurrent I(t) of the current welding point, and/or, reducing the pressingforce when the sign of the expulsion generation exists, therebyreflecting the detected values on the welding electric current and/orthe pressing force in realtime.

[0129]FIG. 12 illustrates a control routine in accordance with the sixthmethod, more particularly, the control method of suppressing theexpulsion generation in the current welding point. The control routineis stored in the control unit 30.

[0130] In the control routine, at step 311, it is determined whether ornot the thermal expansion quantity is equal to or greater than apredetermined value (which is a thermal expansion quantity when asatisfactory welding has been conducted). When the thermal expansionquantity is equal to or a greater than the predetermined value, theroutine proceeds to step 316 where the value of the thermal expansionquantity is stored into the database, and then proceeds to an end step.When it is determined at step 311 that the thermal expansion quantityhas not reached the predetermined value, the routine proceeds to step312 where the current welding electric current is increased by apredetermined value, and the routine then proceeds to step 313. At step313, it is determined whether or not the thermal expansion quantity isbeginning to decrease. When it is determined that the thermal expansionquantity is not beginning to decrease, it is determined that anexpulsion has not been generated and the routine returns to step 311.When it is determined at step 313 that the thermal expansion quantity isbeginning to decrease, the routine proceeds to step 314. At step 314, adifferential value dF/dt (including ΔF/Δt) of the pressing force and/ora differential value dx/dt (including Δx/Δt) of the position of thefixed side welding tip from the beginning of the decrease in the thermalexpansion quantity are calculated and are compared with a predeterminedvalue or values F₀ and/or x₀, at every moment. When the differentialvalues dF/dt and/or dx/dt, which are negative values, are equal to orgreater than the predetermined values F₀ and/or x₀, which are negativevalues, it is determined that the sign of the expulsion generation doesnot exist, and the routine returns to step 311. When the differentialvalues dF/dt and/or dx/dt are smaller than the predetermined values F₀and/or x₀, which means that gradients of curves of the pressing forceand/or the position of the fixed side welding tip suddenly change, it isdetermined that the sign of the expulsion generation exists and theroutine proceeds to step 315, where the welding electric current I(t) ofthe current welding point is decreased or stopped.

[0131] Due to this operation, the expulsion is prevented from beinggenerated in the current welding point.

[0132] A seventh method, a method for controlling a welding strength,will be explained with reference to FIGS. 13 and 14.

[0133] The seventh control method is conducted using the welding gun 1.The welding gun 1 includes a moving side portion 10 including a movingside welding tip 11 and a driving device 12 for driving the moving sidewelding tip 11 and a fixed side portion 20 including a fixed sidewelding tip 21 and an arm 22 supporting the fixed side welding tip 21. Afixed side sensor 23 is disposed in the fixed side portion 20 fordetecting at least one of a position of the fixed side welding tip 21and a pressing force imposed on the fixed side welding tip 21. A movingside sensor 13 is not necessarily disposed in the moving side portion10. The driving device 12 may be a servo motor, or may be an aircylinder.

[0134] The seventh method includes:

[0135] {circle over (1)} determining whether a thermal expansionquantity of the welding portion 61 detected by the fixed side sensor 23is equal to or greater than a predetermined value, ending welding of thecurrent welding point when the thermal expansion quantity is equal to orgreater than the predetermined value;

[0136] {circle over (2)} increasing a welding electric current I(t) whenthe thermal expansion quantity is smaller than the predetermined value;and

[0137] {circle over (3)} decreasing the welding electric current I(t)when a decrease in the thermal expansion quantity is found duringwelding.

[0138]FIG. 14 illustrates a control routine in accordance with theseventh method, more particularly, the method of controlling a weldingstrength. The control routine is stored in the control unit 30.

[0139] In the control routine, at step 321, it is determined whether ornot the thermal expansion quantity is equal to or greater than apredetermined value (which is a predetermined thermal expansion quantitywhen a satisfactory welding has been conducted). When the thermalexpansion quantity is equal to or greater than the predetermined value,the routine proceeds to step 325, where the data of the thermalexpansion quantity is stored into the database, and the routine thenproceeds to an end step. When it is determined at step 321 that thethermal expansion quantity does not reach the predetermined value, theroutine proceeds to step 322, where the welding electric current of thecurrent welding point is increased by a predetermined value ΔI(t), andthe routine then proceeds to step 323. At step 323, it is determinedwhether the thermal expansion quantity is beginning to decrease duringwelding. When it is determined that the thermal expansion quantity isnot beginning to decrease, it is determined that the expulsion has notbeen generated, and the routine returns to step 321. When it isdetermined at step 323 that the thermal expansion quantity is beginningto decrease, the routine proceeds to step 324, where the current weldingelectric current is decreased by a predetermined value ΔI(t)′, and theroutine returns to step 321.

[0140] Due to this operation, a welding having a predetermined thermalexpansion quantity, that is a welding having a necessary weldingstrength can be conducted by a maximum welding electric current and in aminimum period of time, thereby preventing an expulsion from beinggenerated at the current welding point.

[0141] An eighth method, a control method in which when a clearanceexists between workpieces the clearance is eliminated and then weldingis conducted, will be explained with reference with FIGS. 15 and 16.

[0142] The eighth method is conducted using the welding gun 1. Thewelding gun 1 includes a moving side portion 10 including a moving sidewelding tip 11 and a driving device 12 for driving the moving sidewelding tip 11 and a fixed side portion 20 including a fixed sidewelding tip 21 and an arm 22 supporting the fixed side welding tip 21. Afixed side sensor 23 is disposed in the fixed side portion 20 fordetecting at least one of a position of the fixed side welding tip 21and a pressing force imposed on the fixed side welding tip 21. A movingside sensor 13 is disposed in the moving side portion 10 for detectingat least one of a position of the moving side welding tip 11 and apressing force caused in the moving side welding tip 11.

[0143] The eighth method includes:

[0144] {circle over (1)} detecting contacting positions x₁ and x₁′ ofthe moving side welding tip 11 and the fixed side welding tip 21 with aworkpiece 60, respectively, when the moving side welding tip 11 and thefixed welding tip 21 begin to contact the workpiece 60;

[0145] {circle over (2)} calculating differentials (x_(T)−x₁,x_(T)′−x₁′) between objective positions x_(T) and x_(T)′ of the movingside welding tip 11 and the fixed side welding tip 21, respectively,which are previously stored in a welding robot 40, and the contactingpositions x₁ and x₁′, respectively; and

[0146] {circle over (3)} continuing pressing the workpiece 60, whendifferentials exist between the objective positions x_(T) and x_(T)′ andthe contacting positions x₁ and x₁′, respectively, based on gainsproportional to the differentials till the differentials become zero.

[0147]FIG. 16 illustrates a control routine in accordance with theeighth method.

[0148] At step 401, the objective positions x_(T) and x_(T)′ of themoving side welding tip 11 and the fixed side welding tip 21, which arepreviously stored in a RAM (random access memory) of the control unit 30of the welding robot 40, are entered into a CPU (central processingunit). Then, at step 402, the contacting positions x₁ and x₁′ of themoving side welding tip 11 and the fixed side welding tip 21 when themoving side tip 11 and the fixed side tip 21 begin to contact workpieces62 and 63, respectively, are detected by the sensors 13 and 23 and thenentered into the computer.

[0149] At step 403, differentials (x_(T)−x₁, x_(T)′−x₁′) between theobjective positions x_(T) and x_(T)′ and the actual contacting positionsx₁ and x₁′ of the moving side welding tip 11 and the fixed side weldingtip 21 are calculated.

[0150] Then, the routine proceeds to step 404, where it is determinedwhether or not differentials exist between the objective positions x_(T)and x_(T)′ and the contacting positions x₁ and x₁′. When differentialsexist, the routine proceeds to step 405, where pressing the workpieces62 and 63 is continued based on gains proportional to the differentialstill the welding tips move from the contacting positions x₁ and x₁′ tothe objective positions x_(T) and x_(T)′, and the routine then returnsto the calculation step 403. When it is determined at step 404 thatdifferentials do not exist, the routine proceeds to step 406, wherearriving positions x₂ and x₂′ of the moving side welding tip 11 and thefixed side welding tip 21 are obtained from the current detectedpositions of the moving side welding tip 11 and the fixed side weldingtip 21.

[0151] In accordance with the eighth method, differentials between theobjective positions x_(T) and x_(T)′ and the contacting positions x₁ andx₁′ of the moving side welding tip 11 and the fixed side welding tip 21are calculated. When differentials exist, i.e., when a clearance 61exists between the workpieces 62 and 63, pressing the workpieces 62 and63 is continued based on gains proportional to the differentials. Due tothis operation, the workpieces are pressed against each other and theclearance 61 is thus eliminated. Therefore, even if a clearance 61exists between the workpieces 62 and 63, the clearance 61 will beeliminated before welding is conducted, which enables a spot-welding ofa high quality to be performed.

[0152] A ninth method, which is a method of controlling a pressingforce, will be explained with reference to FIGS. 15 and 16.

[0153] The ninth method is conducted using the welding gun 1. Thewelding gun 1 includes a moving side portion 10 including a moving sidewelding tip 11 and a driving device 12 for driving the moving sidewelding tip 11 and a fixed side portion 20 including a fixed sidewelding tip 21 and an arm 22 supporting the fixed side welding tip 21. Afixed side sensor 23 is disposed in the fixed side portion 20 fordetecting at least one of a position of the fixed side welding tip 21and a pressing force imposed on the fixed side welding tip 21. A movingside sensor 13 is disposed in the moving side portion 10 for detectingat least one of a position of the moving side welding tip 11 and apressing force caused in the moving side welding tip 11.

[0154] The ninth method includes:

[0155] {circle over (1)} detecting contacting positions x₁ and x₁′ ofthe moving side welding tip 11 and the fixed side welding tip 21 with aworkpiece 60, respectively, when the moving side welding tip 11 and thefixed welding tip 21 begin to contact the workpiece 60;

[0156] {circle over (2)} calculating differentials (x_(T)−x₁,x_(T)′−x₁′) between objective positions x_(T) and x_(T)′ of the movingside welding tip 11 and the fixed side welding tip 21, respectively,which are previously stored in a welding robot 40, and the contactingpositions x₁ and x₁′, respectively; and

[0157] {circle over (3)} continuing pressing the workpiece 60, whendifferentials exist between the objective positions x_(T) and x_(T)′ andthe contacting positions x₁ and x₁′, respectively, based on gainsproportional to the differentials and returning to the step ofcalculating differentials, while obtaining arriving positions x₂ and x₂′of the moving side welding tip 11 and the fixed side welding tip 21,respectively, from the current detected positions of the moving sidewelding tip 11 and the fixed side welding tip 21 when differentials donot exist;

[0158] {circle over (4)} calculating a pressing force P₀ required forthe moving side welding tip 11 and the fixed side welding tip 21 toreach the arriving positions x₂ and x₂′;

[0159] {circle over (5)} adding a pressing force P₁ necessary forwelding to the pressing force P₀ and imposing the total pressing force:P_(T) which is a summation of P₀ and P₁ on the workpiece 60; and

[0160] {circle over (6)} pressing a welding electric current between themoving side welding tip 11 and the fixed side welding tip 21, therebyconducting welding.

[0161]FIG. 16 illustrates a control routine in accordance with the ninthmethod.

[0162] At step 401, the objective positions x_(T) and x_(T)′ of themoving side welding tip 11 and the fixed side welding tip 21, which arepreviously stored in the RAM of the control unit 30 of the welding robot40, are entered into the CPU. Then, at step 402, the contactingpositions x₁ and x₁′ of the moving side welding tip 11 and the fixedside welding tip 21 when the moving side tip 11 and the fixed side tip21 begin to contact workpieces 62 and 63, respectively, are detected bythe sensors 13 and 23 and then entered into the computer.

[0163] At step 403, differentials (x_(T)−x₁, x_(T)′−x₁′) between theobjective positions x_(T) and x_(T)′ and the actual contacting positionsx₁ and x₁′ of the moving side welding tip 11 and the fixed side weldingtip 21 are calculated.

[0164] Then, the routine proceeds to step 404, where it is determinedwhether differentials exist between the objective positions x_(T) andx_(T)′ and the contacting positions x₁ and x₁′. When differentialsexist, the routine proceeds to step 405, where pressing the workpieces62 and 63 is continued based on gains proportional to the differentialstill the welding tips move from the contacting positions x₁ and x₁′ tothe objective positions x_(T) and x_(T)′, and the routine then returnsto the calculation step 403. When it is determined that differentials donot exist at step 404, the routine proceeds to step 406, where arrivingpositions x₂ and x₂′ of the moving side welding tip 11 and the fixedside welding tip 21 are obtained based on the current detected positionsof the moving side welding tip 11 and the fixed side welding tip 21.

[0165] Then, the routine proceeds to step 407, where a pressing force P₀required for the moving side welding tip 11 and the fixed side weldingtip 21 to reach the arriving positions x₂ and x₂′ is calculated. Thepressing force P₀ is a spring back force of workpieces 62 and 63 causedwhen the moving side welding tip 11 and the fixed side welding tip 21move from the contacting positions x₁ and x₁′ to the arriving positionsx₂ and x₂′, in a case where a clearance 61 exists between the workpieces62 and 63.

[0166] Then, the routine proceeds to step 408, where a pressing force P₁necessary for welding is added to the pressing force P₀ to obtain thetotal pressing force: P_(T)=P₀+P₁. At step 409, it is determined whetheror not the total pressing force P_(T) should be imposed on theworkpieces 62 and 63. When it is determined that the total pressingforce P_(T) should not be imposed, the routine returns to step 408,where the total pressing force P_(T) is recalculated.

[0167] When it is determined at step 409 that the pressing force P_(T)should be imposed on the workpieces 62 and 63, the routine proceeds tostep 410, where the welding electric current is pressed between themoving side welding tip 11 and the fixed side welding tip 21, therebyconducting welding. Welding is conducted for a predetermined period oftime which is set by a timer 411.

[0168] In accordance with the ninth control method, differentialsbetween the objective positions x_(T) and x_(T)′ and the contactingpositions x₁ and x₁′ of the moving side welding tip 11 and the fixedside welding tip 21 are calculated. When differentials exist, i.e., whena clearance 61 exists between the workpieces 62 and 63, pressing theworkpieces is continued based on gains proportional to thedifferentials. Due to this operation, the workpieces 62 and 63 arepressed against each other and the clearance 61 is thus eliminated. Whenthe differentials become zero or the differentials do not exist, thepressing force P₀ required for the moving side welding tip 11 and thefixed side welding tip 21 to reach the arriving positions x₂ and x₂′ iscalculated. The pressing force P₀ is a spring back force of theworkpiece, and is imposed on the workpieces 62 and 63 for the purpose ofremoving the clearance 61 between the workpieces 62 and 63. The pressingforce P₁ necessary for welding is added to the pressing force P₀ and thetotal pressing force P_(T) is imposed on the workpiece. Due to thisoperation, even if a clearance exists between the workpieces, therequired pressing force P₁ can be imposed on the workpieces, whichenables a spot-welding of a high quality having a sufficient pressingforce to be performed.

[0169] A tenth method, which is a method of correcting a track of awelding robot, will be explained with reference to FIGS. 15 and 16.

[0170] The tenth method is conducted using the welding gun 1. Thewelding gun 1 includes a moving side portion 10 including a moving sidewelding tip 11 and a driving device 12 for driving the moving sidewelding tip 11 and a fixed side portion 20 including a fixed sidewelding tip 21 and an arm 22 supporting the fixed side welding tip 21. Afixed side sensor 23 is disposed in the fixed side portion 20 fordetecting at least one of a position of the fixed side welding tip 21and a pressing force imposed on the fixed side welding tip 21. A movingside sensor 13 is disposed in the moving side portion 10 for detectingat least one of a position of the moving side welding tip 11 and apressing force caused in the moving side welding tip 11.

[0171] The tenth method includes:

[0172] {circle over (1)} detecting contacting positions x₁ and x₁′ ofthe moving side welding tip 11 and the fixed side welding tip 21 with aworkpiece 60, respectively, when the moving side welding tip 11 and thefixed welding tip 21 begin to contact the workpiece 60;

[0173] {circle over (2)} calculating differentials (x_(T)−x₁,x_(T)′−x₁′) between objective positions x_(T) and x_(T)′ of the movingside welding tip 11 and the fixed side welding tip 21, respectively,which are previously stored in a welding robot 40, and the contactingpositions x₁ and x₁′, respectively; and

[0174] {circle over (3)} continuing pressing the workpiece, whendifferentials exist between the objective positions x_(T) and x_(T)′ andthe contacting positions x₁ and x₁′, respectively, based on gainsproportional to the differentials and returning to the calculating step,while obtaining arriving positions x₂ and x₂′ of the moving side weldingtip 11 and the fixed side welding tip 21, respectively, from the currentdetected positions of the moving side welding tip 11 and the fixed sidewelding tip 21 when differentials do not exist; and

[0175] {circle over (4)} correcting the objective positions x_(T) andx_(T)′ so that the differentials between the objective positions x_(T)and x_(T)′ and the arriving positions x₂ and x₂′, respectively, becomezero.

[0176]FIG. 16 also illustrates a control routine in accordance with thetenth method for correcting the track of the welding robot.

[0177] At step 401, the objective positions x_(T) and x_(T)′ of themoving side welding tip 11 and the fixed side welding tip 21, which arepreviously stored in the RAM of the control unit 30 of the welding robot40, are entered into the CPU. Then, at step 402, the contactingpositions x₁ and x₁′ of the moving side welding tip 11 and the fixedside welding tip 21 when the moving side tip 11 and the fixed side tip21 contact workpieces 62 and 63, respectively, are detected by thesensors 13 and 23 and then entered to the computer.

[0178] At step 403, differentials (x_(T)−x₁, x_(T)′−x₁′) between theobjective positions x_(T) and x_(T)′ and the real contacting positionsx₁ and x₁′ of the moving side welding tip 11 and the fixed side weldingtip 21 are calculated

[0179] Then, the routine proceeds to step 404, where it is determinedwhether or not differentials exist between the objective positions x_(T)and x_(T)′ and the contacting positions x₁ and x₁′. When differentialsexist, the routine proceeds to step 405, where pressing the workpiece 62and 63 is continued based on gains proportional to the differentials,and the routine then returns to the calculation step 403. When it isdetermined at step 404 that differentials do not exist, the routineproceeds to step 406, where arriving positions x₂ and x₂′ of the movingside welding tip 11 and the fixed side welding tip 21 are obtained basedon the current detected positions of the moving side welding tip 11 andthe fixed side welding tip 21.

[0180] Then, the routine proceeds directly to step 412, or the routineproceeds to step 412 through pressing force control steps 407 to 411.

[0181] In a case where the routine proceeds to step 412 through thepressing force steps 407 to 411, the routine first proceeds to step 407,where a pressing force P₀ required for the moving side welding tip 11and the fixed side welding tip 21 to reach the arriving positions x₂ andx₂′ is calculated. The pressing force P₀ is a spring back force of theworkpiece caused when the moving side welding tip 11 and the fixed sidewelding tip 21 move from the contacting positions x₁ and x₁′ to thearriving positions x₂ and x₂′, in a case where a clearance 61 existsbetween the workpieces 62 and 63.

[0182] The routine proceeds to step 408, where a pressing force P₁necessary for welding is added to the pressing force P₀ to obtain thetotal pressing force: P_(T)=P₀+P₁. At step 409, it is determined whetheror not the total pressing force P_(T) should be imposed on theworkpieces 62 and 63. When it is determined that the total pressingforce P_(T) should not be imposed, the routine returns to step 408,where the pressing force P_(T) is recalculated.

[0183] When it is determined at step 409 that the pressing force P_(T)should be imposed on the workpieces 62 and 63, the routine proceeds tostep 410, where the welding electric current is pressed between themoving side welding tip 11 and the fixed side welding tip 21 therebyconducting welding. Welding is conducted for a predetermined period oftime which is set by timer 411.

[0184] Then, the routine proceeds to step 412, where the objectivepositions x_(T) and x_(T)′ are corrected so that the differentialsbetween the objective positions x_(T) and x_(T)′ and the arrivingpositions x₂ and x₂′, respectively, become zero. Due to this operation,correction of the welding robot track, i.e., correction of the objectivepositions x_(T) and x_(T)′ is performed.

[0185] Then, the routine proceeds to an end step, or the routineproceeds to the end step through steps 413 to 416 when necessary.

[0186] At step 413, when the arriving positions x₂ and x₂′ are expressedin the form of time functions x₂ (t) and x₂′ (t′), a simultaneousarrival of the moving side welding tip 11 and the fixed side welding tip21 to the arriving points is examined by comparing arrival times t andt′, and objective gains and objective positions x_(T) and x_(T)′ arecorrected so that the arrival times t and t′ are equal to each other.

[0187] Then, at step 414, it is determined whether or not confirmationof the above data and correction of the program using the data should beconducted. When the confirmation and correction operation is notnecessary, the routine directly proceeds to the end step, where theroutine ends. When the confirmation and correction operation isnecessary, it is determined at step 415 whether the correction may beconducted, by the operator or by learning control of the control unit30. When the correction may be conducted, the routine proceeds to step416, where the correction is performed. Then, the routine proceeds tostep 417, where the above data including the corrected data are storedinto the database, and the routine proceeds to the end step, where theroutine ends.

[0188] The data stored into the database at step 417 can be used forvarious kinds of managements such as management of a strength at everywelding point of an automobile body and management of weldingdeformation of an automobile body.

[0189] The routine may proceed to step 414 directly from step 403 orstep 412.

[0190] In accordance with the tenth method of correcting the track ofthe welding robot, the objective positions x_(T) and x_(T)′ can becorrected to the real arriving positions x₂ and x₂′.

[0191] Finally, an eleventh method, which is a method of managing achange in a positional accuracy at a welding point, will be explainedwith reference to FIGS. 17-21.

[0192] The eleventh method is conducted using the welding gun 1. Thewelding gun 1 includes a moving side portion 10 including a moving sidewelding tip 11 and a driving device 12 for driving the moving sidewelding tip 11 and a fixed side portion 20 including a fixed sidewelding tip 21 and an arm 22 supporting the fixed side welding tip 21. Afixed side sensor 23 is disposed in the fixed side portion 20 fordetecting at least one of a position of the fixed side welding tip 21and a pressing force imposed on the fixed side welding tip 21. A movingside sensor 13 is disposed in the moving side portion 10 for detectingat least one of a position of the moving side welding tip 11 and apressing force caused in the moving side welding tip 11.

[0193] The eleventh method includes:

[0194] {circle over (1)} entering positional information x₁, x₁′, x₂ andx₂′ from a database storing, at steps 306, 316, 325, 417, etc.,contacting positions x₁ and x₁′ of the moving side welding tip 11 andthe fixed side welding tip 21 with a workpiece 60 at a time when themoving side welding tip 11 and the fixed side welding tip 21 begin tocontact the workpiece 60, and arriving positions x₂ and x₂′ of themoving side welding tip 11 and the fixed side welding tip 21 at a timewhen the moving side welding tip 11 and the fixed side welding tip 21have fully pressed the workpiece 60;

[0195] {circle over (2)} calculating a positional accuracy matrix(vector) of each station 70 with respect to a plurality of stations 70each having at least one robot 40, the matrix being defined by thefollowing:

|Φ_(n)|=[|P₁|, |P₂|, . . . , |P_(m)|]

[0196] wherein,

[0197] n: No. n station (that is, a station number of the currentstation)

[0198] m: the number of robots equal to or greater than 1, of the No. nstation

[0199] |P_(j)|: a positional accuracy matrix of a robot (No. j robot),obtained from welding points P₁, P₂, . . . , and P_(k) and positions x₁,x₁′, x₂, and x₂′ of the robot; and

[0200] {circle over (3)} managing a positional accuracy change of thewelding point of the workpiece based on a value and/or values definedby: |Φ_(n)|−|Φ_(n−1)| and/or |Φ_(n)|−|Φ₁ |.

[0201] Each matrix will be explained more in detail.

[0202] In a case where each welding robot 40 is a six-articulation-typerobot having a seven axis constituted by a welding gun axis (FIG. 17),the positional accuracy matrix |P_(j)| is defined by a matrixillustrated in FIG. 18. The matrix |P_(j)| is a composite matrix of apositional accuracy matrix |M₆| of the fixed side welding tip 21 and apositional accuracy matrix |M₇| of the moving side welding tip 11. Thepositional accuracy matrix |M₆| of the fixed side welding tip 21 isidentical to a positional matrix of a tip end of the six-articulation ofthe robot. The positional accuracy matrix |M₇| of the moving sidewelding tip 11 is identical to a positional matrix of a tip end of thewelding gun axis. When references P₁, P₂, . . . , and P_(k) expressrespective welding points of each robot, the matrix |M₆| is made byarranging the contacting positions x₁′ of the fixed side welding tip 21with the workpiece at the welding points in a column and arranging thearriving positions x₂′ of the fixed side welding tip at the weldingpoint in a column, and the matrix |M₇| is made by arranging thecontacting positions x₁ of the moving side welding tip 11 with theworkpiece at the welding points in a column and the arriving position x₂of the moving side welding tip at the welding points in a column.

[0203] As illustrated in FIG. 19, in a case where robots are provided bym in number (where m is an integer equal to or greater than 1) ispositioned at No. n welding station, a positional accuracy matrix orvector of the station 70 is made by arranging the positional accuracymatrices of the welding robot |P₁|, |P₂|, . . . , |P_(m)| of the No. nstation in a row, that is, is expressed by the following equation:

|Φ_(n)|=[|P₁|, |P₂|, . . . , |P_(m)|]

[0204] Then, by calculating a differential (|Φ_(n)|−|Φ_(n−1)|) betweenthe positional accuracy matrix of No. n station and the positionalaccuracy matrix of the previous No. n-1 station, a deformation of theworkpiece caused between No.n station and No.n-1 station due tospot-welding is obtained. Further, by calculating a differential(|Φ_(n)|−|Φ₁|), a deformation of the workpiece caused between No. 1station and No. n station due to spot-welding is obtained.

[0205]FIG. 20 illustrates a routine of calculating a deformation of theworkpiece due to spot-welding. At step 501, positional data x₁, x₁′, x₂,and x₂′ at the respective welding points are entered from the databasewhich has stored those data at step 417 of FIG. 16. Then, at step 502,the positional accuracy matrix |P_(m)| of each welding robot 40 is made,and, at step 503, the positional accuracy matrix |Φ_(n)| of each station70 is made. Then, at step 504, differentials |Φ_(n)|−|Φ_(n−1)| and|Φ_(n)|−|Φ_(n)| are calculated, respectively. At step 505, it isdetermined whether or not the calculated value (|Φ_(n)|−|Φ_(n−1)|) issmaller than a predetermined allowable deformation |Φ_(s)| between thestations. When the deformation is equal to or greater than the allowabledeformation, a warning is issued. Similarly, at step 506, it isdetermined whether or not the calculated value |Φ_(n)|−|Φ₁| is smallerthan a predetermined allowable deformation |Φ₁|. When the deformation isequal to or greater than the allowable deformation quantity, a warningis issued. No warnings mean that welding deformation of the workpiecehas been satisfactorily suppressed. Due to this operation, managing of awelding deformation is possible.

[0206] Further, the positional accuracy matrix |Φ_(n)| of the station 70is also available to, for example, a method of determining an optimumwelding order which makes a deformation of the workpiece minimum. FIG.21 illustrates an example of the welding order management. At step 510,the welding order is variously changed, and with each welding order,|Φ_(n)|−|Φ₁| is calculated. At step 511, the minimum one is selectedfrom a plurality of values of |Φ_(n)|−|Φ₁|. At step 512, it isdetermined, by the operator or by the computer based on learning,whether or not the current welding order should be changed to thatwelding order having the minimum value of |Φ_(n)|−|Φ₁|. When it isdetermined that the welding order should be changed, the routineproceeds to step 513, where the welding order is changed. When it isdetermined that the welding order should not be changed, the routinereturns to step 510, where the welding order is further changed and theroutine is repeated.

[0207] According to the welding gun of the present invention, thefollowing technical advantages are obtained:

[0208] First, since a sensor is provided in the fixed side portion, thesensor can be disposed in a portion of the welding gun where themechanical impedance is smaller than that of the moving side portion andwhere it is not disposed via a gear such as a speed reducer from thewelding tip. As a result, a displacement of the welding tip and apressing force imposed on the welding tip can be detected with highaccuracy and a good response. By controlling the welding gun accordingto the output of the sensor, a scope of objects capable of being,controlled is widened.

[0209] Second, since the mechanical impedance of the fixed side portionis set in a range where the sensor can effectively detect a displacementof the fixed side welding tip and the pressing force, the mechanicalimpedance of the fixed side portion can remain small, unlike aconventional welding gun in which the mechanical impedance of a fixedside portion is increased to be nearly equal to a mechanical impedanceof the moving side portion. As a result, the arm supporting the fixedside welding tip can be decreased both in rigidity and in size ascompared with the conventional welding gun, which makes the welding guncompact and lightweight.

[0210] Third, since the fixed side sensor is any one of a force sensor(a load sensor), an optical distance sensor (a displacement sensor) anda sensor using an optical fiber, a commercial sensor can be used.

[0211] Fourth, in the case where sensors are provided both in the fixedside portion and the moving side portion of the welding gun, the fixedside sensor and the moving side sensor constitute a redundant sensormeasurement system.

[0212] According to various kinds of methods conducted using theabove-described welding gun, the following technical advantages areobtained:

[0213] In accordance with the first method, that is a method ofcalibrating a sensor conducted using the welding gun, since thereference point of the fixed side sensor is calibrated in a statewherein the moving side welding tip is released from the fixed sidewelding tip, the reference point of the fixed side sensor can becalibrated based on an output of the moving side sensor.

[0214] In accordance with the second method, that is a method ofcalibrating a sensor conducted using the welding gun, since the movingside welding tip is pressed against the fixed side welding tip and thegains of the moving side sensor and the fixed side sensor are adjusted,one sensor can calibrate the other sensor.

[0215] In accordance with the third method, that is a method ofcontrolling welding conducted using the welding gun, a re-weldingfeedback control can be conducted.

[0216] In accordance with the fourth method, that is a method ofcontrolling welding conducted using the welding gun, expulsiongeneration at the corresponding welding point in the next cycle can besuppressed.

[0217] In accordance with the fifth method, that is a method of managinga welding quality conducted using the welding gun, since data about theexpansion quantity, information about whether re-welding has beenperformed and information about whether expulsion has been generated arestored into the memory at every welding point and are periodicallystored into the managing system of a higher level, a welding quality canbe managed.

[0218] In accordance with the sixth method, that is a method ofcontrolling welding conducted using the welding gun, expulsiongeneration at the current welding point can be suppressed.

[0219] In accordance with the seventh method, that is a method ofcontrolling welding conducted using the welding gun, spot welding havinga necessary welding strength can be conducted in a minimum time periodunder a condition that expulsion is not generated, whereby the weldingstrength can be controlled.

[0220] In accordance with the eighth method, that is a method ofcontrolling welding conducted using the welding gun, a clearance betweenthe workpieces is eliminated and welding is then conducted, so that spotwelding of a high quality can be performed.

[0221] In accordance with the ninth method, that is a method ofcontrolling a pressing force of welding conducted using the welding gun,even if a clearance exists between the workpieces, the pressing force P₁necessary for welding can be imposed on the workpieces, so that spotwelding of a high quality having a sufficient pressing force can beperformed.

[0222] In accordance with the tenth method, that is a method ofcorrecting a track of a welding robot conducted using the welding gun,the objective positions x_(T), x_(T)′ can be corrected to the realarriving positions x₂, x₂′, whereby the track of the welding robot ismodified and is prepared for welding of the corresponding welding pointin the next cycle.

[0223] In accordance with the eleventh method, that is a method ofcontrolling a change in a positional accuracy change at a welding pointconducted using the welding gun, the positional accuracy matrix of eachstation |Φ_(n)| is calculated and a deformation of the workpiece can bemanaged.

[0224] Although the present invention has been described with referenceto specific exemplary embodiments, it will be appreciated in the artthat various modifications and alterations can be made to the particularembodiments shown, without materially departing from the novel teachingsand advantages of the present invention. Accordingly, it is to beunderstood that all such modifications and alterations are includedwithin the spirit and scope of the present invention as defined by thefollowing claims.

What is claimed is:
 1. A welding gun comprising: a moving side portionincluding a moving side welding tip and a driving device for drivingsaid moving side welding tip; and a fixed side portion including a fixedside welding tip and an arm supporting said fixed side welding tip,wherein a fixed side sensor for detecting at least one of a position ofsaid fixed side welding tip and a pressing force imposed on said fixedside welding tip is provided in said fixed side portion.
 2. A weldinggun according to claim 1 , wherein a mechanical impedance of said fixedside portion is smaller than that of said moving side portion, saidmechanical impedance of said fixed side portion being set in a rangewhere said fixed side sensor can effectively detect said at least one ofthe position of said fixed side welding tip and the pressing forceimposed on said fixed side welding tip.
 3. A welding gun according toclaim 1 , wherein said fixed side sensor is any one of a force sensor,an optical distance sensor and a sensor using an optical fiber.
 4. Awelding gun according to claim 1 , wherein a moving side sensor fordetecting at least one of a position of said moving side welding tip anda pressing force caused in said moving side welding tip is provided insaid moving side portion, said fixed side sensor and said moving sidesensor constituting a redundant sensor measurement system.
 5. A methodof calibrating a sensor conducted using a welding gun which includes (1)a moving side portion including a moving side welding tip and a drivingdevice for driving said moving side welding tip and (2) a fixed sideportion including a fixed side welding tip and an arm supporting saidfixed side welding tip, wherein a fixed side sensor for detecting atleast one of a position of said fixed side welding tip and a pressingforce imposed on said fixed side welding tip is provided in said fixedside portion, and a moving side sensor for detecting at least one of aposition of said moving side welding tip and a pressing force caused insaid moving side welding tip is provided in said moving side portion,said method comprising: releasing said moving side welding tip from aposition of contact with said fixed side welding tip; and calibrating atleast one of a pressing force information and a positional information areference point of the fixed side sensor.
 6. A method of calibrating asensor conducted using a welding gun which includes (1) a moving sideportion including a moving side welding tip and a driving device fordriving said moving side welding tip and (2) a fixed side portionincluding a fixed side welding tip and an arm supporting said fixed sidewelding tip, wherein a fixed side sensor for detecting at least one of aposition of said fixed side welding tip and a pressing force imposed onsaid fixed side welding tip is provided in said fixed side portion, anda moving side sensor for detecting at least one of a position of saidmoving side welding tip and a pressing force caused in said moving sidewelding tip is provided in said moving side portion, said methodcomprising: increasing pressure of said moving side welding tip againstsaid fixed side welding tip and plotting a pressing force and/or apositional information to obtain characteristic curves of said movingside sensor and said fixed side sensor using a method of least squares;and determining gains of said sensors such that the gains of said movingside sensor and said fixed side sensor are equal to each other.
 7. Acontrol method of welding a workpiece conducted using a welding gunwhich includes (1) a moving side portion including a moving side weldingtip and a driving device for driving said moving side welding tip and(2) a fixed side portion including a fixed side welding tip and an armsupporting said fixed side welding tip, wherein a fixed side sensor fordetecting at least one of a position of said fixed side welding tip anda pressing force imposed on said fixed side welding tip is provided insaid fixed side portion, said method comprising: determining whether ornot an expansion quantity of a welding portion of the workpiece detectedby said fixed side sensor is equal to or greater than a predeterminedvalue, and ending welding of the current welding point when theexpansion quantity is equal to or greater than said predetermined value;increasing the welding electric current when the expansion quantity issmaller than the predetermined value; counting the number of times thewelding electric current is increased and determining whether re-weldingprogram should be conducted when said number of times exceeds apredetermined number, and ending welding at the current welding pointwhen it is determined that the re-welding program should not beconducted; conducting re-welding when it is determined that there-welding program should be conducted; and determining, when re-weldingis conducted, whether or not the expansion quantity of the weldingportion of the workpiece during re-welding is equal to or greater thanthe predetermined value, ending welding of the current welding pointwhen the expansion quantity is equal to or greater than thepredetermined value, while issuing a warning when the expansion quantitydoes not reach the predetermined value.
 8. A control method of welding aworkpiece conducted using a welding gun which includes (1) a moving sideportion including a moving side welding tip and a driving device fordriving said moving side welding tip and (2) a fixed side portionincluding a fixed side welding tip and an arm supporting said fixed sidewelding tip, wherein a fixed side sensor for detecting at least one of aposition of said fixed side welding tip and a pressing force imposed onsaid fixed side welding tip is provided in said fixed side portion, saidmethod comprising: obtaining an expansion quantity of a welding portionof the workpiece, a position and a pressing force of said fixed sidewelding tip, from detected values detected at every moment by said fixedside sensor; determining whether or not an expulsion is generated in thewelding portion by comparing at least one of a value of the pressingforce and the position of said fixed side welding tip at a point whenthe expansion quantity begins to decrease with at least one of a valueof the pressing force and the position of said fixed side welding tipafter a predetermined period of time has passed from the beginning ofthe decrease in the expansion quantity; and setting a welding electriccurrent of a corresponding welding point in a next cycle to be equal toor greater than a welding electric current of the current welding pointwhen expulsion is not generated, while setting the welding electriccurrent of the corresponding welding point in the next cycle to besmaller than the welding electric current of the current welding pointwhen the expulsion is generated, thereby reflecting the data of thecurrent cycle on a welding condition of the next cycle.
 9. A method ofmanaging a welding quality conducted using a welding gun which includes(1) a moving side portion including a moving side welding tip and adriving device for driving said moving side welding tip and (2) a fixedside portion including a fixed side welding tip and an arm supportingsaid fixed side welding tip, wherein a fixed side sensor for detectingat least one of a position of said fixed side welding tip and a pressingforce imposed on said fixed side welding tip is provided in said fixedside portion, said method comprising: storing data about an expansionquantity, information about whether re-welding has been conducted, andinformation about whether an expulsion has been generated into a memoryat each welding point after welding of the each welding point has beeconducted, and periodically storing the data into a managing system of ahigher level.
 10. A control method of welding a workpiece conductedusing a welding gun which includes (1) a moving side portion including amoving side welding tip and a driving device for driving said movingside welding tip and (2) a fixed side portion including a fixed sidewelding tip and an arm supporting said fixed side welding tip, wherein afixed side sensor for detecting at least one of a position of said fixedside welding tip and a pressing force imposed on said fixed side weldingtip is provided in said fixed side portion, said method comprising:obtaining an expansion quantity of a welding portion of the workpiece, aposition of said fixed side welding tip, a differential value of theposition, a pressing force, and a differential value of the pressingforce, which change at every moment, from detected values detected atevery moment by said fixed side sensor; determining whether a sign of anexpulsion generation exists in the welding portion by comparing at leastone of the differential value of the position of said fixed side weldingtip and the differential value of the pressing force from the beginningof a decrease in said expansion quantity with a predetermined value atevery moment; and decreasing or stopping the welding electric currentwhen the sign of the expulsion generation exists, thereby reflecting thedetected values on the welding electric current in realtime.
 11. Acontrol method of welding a workpiece conducted using a welding gunwhich includes (1) a moving side portion including a moving side weldingtip and a driving device for driving said moving side welding tip and(2) a fixed side portion including a fixed side welding tip and an armsupporting said fixed side welding tip, wherein a fixed side sensor fordetecting at least one of a position of said fixed side welding tip anda pressing force imposed on said fixed side welding tip is provided insaid fixed side portion, said method comprising: obtaining an expansionquantity of a welding portion of the workpiece, a position of said fixedside welding tip, a differential value of the position, a pressingforce, and a differential value of the pressing force, which change atevery moment, from detected values detected at every moment by saidfixed side sensor; determining whether a sign of an expulsion generationexists in the welding portion by comparing at least one of thedifferential value of the position of said fixed side welding tip andthe differential value of the pressing force from the beginning of adecrease in said expansion quantity with a predetermined value at everymoment; and reducing the pressure force when the sign of the expulsiongeneration exists, thereby reflecting the detected values on thepressing force in realtime.
 12. A control method of welding a workpiececonducted using a welding gun which includes (1) a moving side portionincluding a moving side welding tip and a driving device for drivingsaid moving side welding tip and (2) a fixed side portion including afixed side welding tip and an arm supporting said fixed side weldingtip, wherein a fixed side sensor for detecting at least one of aposition of said fixed side welding tip and a pressing force imposed onsaid fixed side welding tip is provided in said fixed side portion, saidmethod comprising: determining whether an expansion quantity of awelding portion of the workpiece detected by said fixed side sensor isequal to or greater than a predetermined value, and ending welding ofthe current welding point when the expansion quantity is determined tobe equal to or greater than the predetermined value; increasing awelding electric current when the expansion quantity is smaller than thepredetermined value; and decreasing the welding electric current when adecrease in the expansion quantity is found during welding.
 13. Acontrol method of a pressing force of welding conducted using a weldinggun which includes (1) a moving side portion including a moving sidewelding tip and a driving device for driving said moving side weldingtip and (2) a fixed side portion including a fixed side welding tip andan arm supporting said fixed side welding tip, wherein a fixed sidesensor for detecting at least one of a position of said fixed sidewelding tip and a pressing force imposed on said fixed side welding tipis provided in said fixed side portion, and a moving side sensor fordetecting at least one of a position of said moving side welding tip anda pressing force caused in said moving side welding tip is provided insaid moving side portion, said method comprising: detecting contactingpositions x₁ and x₁′ of said moving side welding tip and said fixed sidewelding tip, respectively, with a workpiece when said moving sidewelding tip and said fixed welding tip begin to contact the workpiece;calculating differentials between objective positions x_(T) and x_(T)′of said moving side welding tip and said fixed side welding tip,respectively, which are previously stored in a welding robot, and saidcontacting positions x₁ and x₁′, respectively; and continuing pressingthe workpiece, when differentials exist between said objective positionsx_(T) and x_(T)′ and said contacting positions x₁ and x₁′, respectively,based on gains proportional to said differentials until saiddifferentials become zero.
 14. A control method of a pressing force ofwelding conducted using a welding gun which includes (1) a moving sideportion including a moving side welding tip and a driving device fordriving said moving side welding tip and (2) a fixed side portionincluding a fixed side welding tip and an arm supporting said fixed sidewelding tip, wherein a fixed side sensor for detecting at least one of aposition of said fixed side welding tip and a pressing force imposed onsaid fixed side welding tip is provided in said fixed side portion, anda moving side sensor for detecting at least one of a position of saidmoving side welding tip and a pressing force caused in said moving sidewelding tip is provided in said moving side portion, said methodcomprising: detecting contacting positions x₁ and x₁′ of said movingside welding tip and said fixed side welding tip with a workpiece,respectively, when said moving side welding tip and said fixed weldingtip begin to contact the workpiece; calculating differentials betweenobjective positions x_(T) and x_(T)′ of said moving side welding tip andsaid fixed side welding tip, respectively, which are previously storedin the welding robot, and said contacting positions x₁ and x₁′,respectively; continuing pressing the workpiece, when differentialsexist between said objective positions x_(T) and x_(T)′ and saidcontacting positions x₁ and x₁′, respectively, based on gainsproportional to said differentials and returning to said calculatingdifferentials, while obtaining arriving positions x₂ and x₂′ of saidmoving side welding tip and said fixed side welding tip, respectively,from the current detected positions of said moving side welding tip andsaid fixed side welding tip when differentials do not exist; calculatinga pressing force P₀ required for said moving side welding tip and saidfixed side welding tip to reach said arriving positions; adding apressing force P₁ necessary for welding to said pressing force P₀ andimposing the total pressing force P_(T) which is a summation of said P₀and P₁ on the workpiece; and pressing a welding electric current betweensaid moving side welding tip and said fixed side welding tip, therebyconducting welding.
 15. A method of correcting a track of a weldingrobot conducted using a welding gun which includes (1) a moving sideportion including a moving side welding tip and a driving device fordriving said moving side welding tip and (2) a fixed side portionincluding a fixed side welding tip and an arm supporting said fixed sidewelding tip, wherein a fixed side sensor for detecting at least one of aposition of said fixed side welding tip and a pressing force imposed onsaid fixed side welding tip is provided in said fixed side portion, anda moving side sensor for detecting at least one of a position of saidmoving side welding tip and a pressing force caused in said moving sidewelding tip is provided in said moving side portion, said methodcomprising: detecting contacting positions x₁ and x₁′ of said movingside welding tip and said fixed side welding tip with a workpiece,respectively, when said moving side welding tip and said fixed weldingtip begin to contact the workpiece; calculating differentials betweenobjective positions x_(T) and x_(T)′ of said moving side welding tip andsaid fixed side welding tip, respectively, which are previously storedin the welding robot, and said contact positions x₁ and x₁′,respectively; continuing pressing the workpiece, when differentialsexist between said objective positions x_(T) and x_(T)′ and said contactpositions x₁ and x₁′, respectively, based on gains proportional to saiddifferentials and returning to said calculating differentials, whileobtaining arriving positions x₂ and x₂′ of said moving side welding tipand said fixed side welding tip, respectively, from current detectedpositions of said moving side welding tip and said fixed side weldingtip, when differentials do not exist; and correcting said objectivepositions so that said differentials between said objective positionsx_(T) and x_(T)′ and said arrival positions x₂ and x₂′, respectively,become zero.
 16. A method of managing a change in a positional accuracyof a welding point conducted using a welding gun which includes (1) amoving side portion including a moving side welding tip and a drivingdevice for driving said moving side welding tip and (2) a fixed sideportion including a fixed side welding tip and an arm supporting saidfixed side welding tip, wherein a fixed side sensor for detecting atleast one of a position of said fixed side welding tip and a pressingforce imposed on said fixed side welding tip is provided in said fixedside portion, and a moving side sensor for detecting at least one of aposition of said moving side welding tip and a pressing force caused insaid moving side welding tip is provided in said moving side portion,said method comprising: entering positional information x₁, x₁′, x₂ andx₂′ from a database storing contacting positions x₁ and x₁′ of saidmoving side welding tip and said fixed side welding tip with a workpieceat a time when said moving side welding tip and said fixed side weldingtip begin to contact the workpiece, and arriving positions x₂ and x₂′ ofsaid moving side welding tip and said fixed side welding tip at a timewhen said moving side welding tip and said fixed side welding tip havepressed the workpiece; calculating a positional accuracy vector of astation with respect to a plurality of stations each having at least onerobot, the matrix being defined by the following: |Φ_(n)|=[|P₁|, |P₂|, .. . , |P_(m)|] wherein, n: a station number of the current station m:the number of robots equal to or greater than 1, of station n |P_(j)|: apositional accuracy matrix of a robot (No. j robot), obtained from thewelding points P₁, P₂, . . . , and P_(k) and positions x₁, x₁′, x₂, andx₂′ of the robot; and managing a change in a positional accuracy of thewelding point of the workpiece based on a value and/or values:|Φ_(n)|−|Φ_(n−1)| and/or |Φ_(n)|−|Φ₁|.